Personally, if talking to another person, I would look down or away if formulating (or trying to give the impression that I'm formulating) an answer. Rather than look at the person uncomfortably (on the part of the other person) for 21 seconds -- maybe that's because in person, if you look at someone and say nothing for a long period it invites them to fill the gap and say something instead.
Perhaps it's different or he thinks about it differently because he's looking at a camera? Or it's a one-way press conference?
It does just look odd though. Not to say he shouldn't take time to answer important questions -- and in fact more public figures should do this. But I would just naturally think he would look down or away from the camera while doing it, to indicate that some response is coming during the silence.